Alt text is one of the most consistently mishandled elements of web accessibility. It is either missing entirely, filled with keyword-stuffed phrases, or written as a literal description that misses the point of why the image is there.
The context question
The most important question when writing alt text is: why is this image here? A product photograph communicates what the product looks like — describe it accurately. A decorative image communicates nothing — use alt="" so screen readers skip it.
What to avoid
Avoid starting alt text with "image of" or "photo of." Screen readers already announce the element is an image. Avoid keyword stuffing. Avoid being too literal — "A woman sitting at a desk" is less useful than "A customer service representative reviewing a support ticket on a laptop."
A practical test
Read the alt text aloud without looking at the image. Does it give you enough information to understand what the image is communicating? If yes, it is good alt text.